This invention relates to positive temperature coefficient resistor (PTCR) packages and more particularly to such a package containing a low cost electroded PTCR slug that is pressure contacted by low thermal conductivity metal pieces.
For example it is known to employ a thermally independent PTCR in series with the start winding of a single phase motor whereby the motor is aided in starting by a current flowing in the start winding through the low "cold" resistance of the series connected PTCR. After a short time this current heats the PTCR to the anomaly temperature of the PTCR at which point the PTCR resistance abruptly increases several orders of magnitude causing the current to be diminished accordingly. This is designed to occur just prior to the motor having achieved normal running speed.
A reduced thermal conductivity in a PTCR package leads to reduced power consumption during the steady state running condition, a factor of major importance, e.g. for application in refrigerators, and air conditioners. In PTCR packages designed to handle large currents, e.g. greater than 12 amps., the necessarily heavier electrical conductors between the electroded slug and the package terminals represent a substantial portion of the thermal path between slug and package environment.
This explains, at least in part, why certain manufacturers of PTCR packages have chosen to use stainless steel contacts, stainless steel having a very low thermal conductivity, i.e. 0.08 K cal/sec m .degree.C., compared to that of other metals such as aluminum, copper and carbon steel (0.99, 1.66, and 0.23 K cal/sec m .degree.C., respectively). Beryllium copper also has a low thermal conductivity (0.05) but at least because of associated toxicity problems in its manufacture, berryllium copper is much more (about 3.times. more) expensive than stainless steel.
However, as will be further elaborated herein, pressure contacts between stainless steel and any of the conventional ceramic electrode metals tend to deteriorate rapidly in a high current carrying PTCR package.
Therefore it is an object of the present invention to provide a reliable PTCR package including stainless steel pressure contacts to the electroded PTCR slug.
It is a further object of this invention to provide a method for making such a package, and more particularly the steps for making suitable electrodes that lead to long life in high current PTCR service.